This invention relates to cement compositions comprising ion leachable inorganic compounds preferably in the form of glasses. In particular, this invention relates to providing additives, hereinafter modifying agents, for altering and controlling the reaction time associated with such cement compositions.
Ion leachable inorganic compounds such as the oxides of aluminum, zinc, magnesium, and calcium have been intermixed with other components such as silicone and formed into glasses which, when combined with such hydrogen donating compounds such as acids, will set up into a cementitious mass. The mechanisms for the reaction has been described by Alan D. Wilson, et al. (Journal of Dental Research, Volume 58, No. 3 at pps. 1065-1071, March 1979), and can be represented by the generic equation:
______________________________________ MO + H.sub.2 A = MA + H.sub.2 O ion-leachable Proton Salt inorganic donating Hydrogel compound compound ______________________________________
Cements utilizing this mechanism have generally taken the form of glass powders incorporating the ion leachable inorganic. These are reacted with acid solutions such as aqueous, poly(carboxylic acid) solutions to form a salt hydrogel structure which sets up to a hard mass. Such cement forming compositions have been suggested for use in applications such as dental cements and for orthopedic purposes; i.e., casts and splints. For example, a fluoro alumino silicate glass powder has been suggested for use as the ion leachable component for dental cement as in British Pat. No. 1,316,129. Similarly, such a composition has been suggested for use in orthopedic surgery as in U.S. Pat. Nos. 4,143,018 and 4,043,327.
When using such compositions for orthopedic purposes, for example, certain criteria must be met. The composition, when rendered reactive, must be capable of providing sufficient "working time," i.e., sufficient time from the start of mixing the reactants to allow the doctor time to apply and mold the cast into shape before the material reaches a stage where it is no longer maleable. Generally, such times should be at least about 2 minutes and preferably from about 4 to about 6 minutes.
At the end of the working time, it is most desirable that the cast set to a rock-like state as quickly as possible. While most cements, even after obtaining a rock-like appearance, do not reach their ultimate strength for long periods of time, the material should reach sufficient compressive strength to allow a patient to leave the doctor's office i.e. should be sufficiently hard enough to preclude deformation under expected stresses. This period is referred to as the "setting time" and should be about 6 to about 15 minutes after the cast is applied.
Prior workers in the field have discovered that the rate of setting; i.e., the working and setting times, can be greatly affected by the addition of certain agents which have alternatively been referred to in the art as chelating agents, complexing agents, accelerating agents, or the like. (See for example, Wilson, et al., Journal of Dental Research, Volume 55 No. 3, Pages 489-495, 1976; Crisp, et al., Journal of Dental Research, Volume 55 No. 6, Pages 123-131, 1976; Crisp, et al., Journal of Dentistry, Volume 7, No. 4, Pages 304-319, 1979).
As has been described in the aforementioned U.S. Pat. No. 4,043,327 one way of varying the setting times of these compositions is to add to the mixture an inorganic dicarboxylic or hydrocarboxylic acid. It is stated that this addition appears to exert a chelating effect on calcium ions produced when water is added. Such acids are described in this patent as including tartaric, succinic, oxalic, citric, ascorbic, gluconic or adipic acids. In my co-pending patent application filed on this same day, I have disclosed the use of a particular form of tartaric acid; namely, tartaric acid including a major portion of the racemic mixture of the optically neutral d,l-isomer, which has particular advantage as an accelerating agent.
Unfortunately, it has now been discovered that there is one major drawback in providing compositions including such modifying or accelerating agents as those enumerated above. Specifically, when employing these compositions in the manner such as an orthopedist might, the compositions are generally in the dry form adhered to certain substrates such as gauze, for example. The practitioner prepares the composition for application by first dipping the entire bandage in a bucket or other container of water thereby beginning the gelation reaction. He then molds the cast onto the limb of the patient and allows it to set. The problem encountered with the aforementioned accelerating agents is that they are all, to a great degree, water soluble and if the practitioner follows his usual practice, the compositions are introduced into an environment which contains a large excess of water. Depending upon how long the composition resides in the bucket of water, more or less of the accelerating agent will be dissolved into the large excess of water and become unavailable for the gelation reaction, particularly after the bandage is removed from the water and placed onto the limb of the patient. This variation in solubilization of the modifying agent produces a concommitant variation in the performance of the product. Stated in other words, depending on the idiosyncrasies and methods of the practitioner, the setting and working times of a given composition will vary in an uncontrolled manner. Accordingly, there is a need for providing cementitious compositions with modifying agents which are not greatly affected by the variation in their use.